Is there a way to prevent users from approving their own JE via the standard Approve Journal Entries page? I already tried created by = current user + before record submit but it does not work. The context is also UI when approving via that standard page.

use a approval workflow - use a custom field or workflow field to set the creator. then use to show approval button using the following condition : user not equal to creator. Need to turn on the approval routing for JE in accounting preferences

Sorry, I wasn't very clear. The button and custom field works for us. But there's also a native bulk approval page where that workflow logic doesn't execute.

If you turn on the approval routing and a workflow then it should work. Have done that for so many instances now, it gotta work. Another option is to use suiteapproval bundle - free to use.

For JEs, we just use the role-based approval process so that we can keep the native bulk approval page. We have a workflow that hides the Approve button when the current user and the creator of the JE are the same. That works fine. But they can bypass it when they are approving via the standard bulk approval page of NS.

You should be able to create a workflow that runs on Approve event and returns a user error if the approver and creator are the same. I did a POC for something similar with return authorizations